Water-absorbent resin particles have been used for hygienic materials such as disposable diapers and sanitary articles, for agricultural and gardening materials such as water-retaining materials and soil conditioners, and for industrial materials such as water blocking materials for cables, and dew-catchers. In recent years, in addition to these materials, water-absorbent resin particles are being applied in an even wider range of fields including animal excreta treatment materials such as pet sheets and dog or cat toilet blending materials, portable toilets, aromatic agents, meat drip absorbing sheets, moisturizing cosmetic formulations, and the like. The performance that is demanded for water-absorbent resin particles to be used for such purposes includes high water-absorption capacity, excellent water-absorption rate and particle size suited for the purpose.
Among these, water-absorbent resins used for purposes such as special hygiene materials such as adult diapers, incontinence pads, toilet-training pants and extra-high-absorbent napkins, water blocking materials for cables, pet sheets, portable toilets and the like, are assumed to be subjected to forceful excretion of relatively large amounts of body fluids. For such purposes, particular attention has been directed toward improving absorption capacity (water-absorption capacity) and water-absorption rate. Absorption capacity can be adjusted by the amount of water-absorbent resin particles used, but water-absorption rate is usually based on the characteristic properties of the water-absorbent resin particles. Much research has therefore been carried out in the past with the aim of achieving excellent water-absorption rates for water-absorbent resin particles.
For example, there is disclosed a method of aqueous liquid polymerization in which a porous resin obtained in the presence of a foaming agent is crosslinked near the surface (see Patent document 1).
There are also disclosed, a reversed-phase suspension polymerization method wherein an acrylic acid/acrylic acid salt aqueous liquid is suspended in an alicyclic or aliphatic hydrocarbon solvent in the presence of an HLB 8-12 surfactant, and the acrylic acid/acrylic acid salt is subjected to reversed-phase suspension polymerization (see Patent document 2), and a method of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of water-absorbent resins with different water-absorption rates (see Patent document 3).